As methods for the synthesis of diamond, there have heretofore been practically adopted the method for the direct conversion of graphite including the method which effects the synthesis under an ultrahigh pressure in the presence of an iron or nickel catalyst and the method which accomplishes the synthesis with the aid of an explosive.
In recent years, as a low-pressure CVD method, the method which attains synthesis of diamond by setting a hydrocarbon or a mixed gas of hydrogen and an organic compound containing nitrogen, oxygen, etc. into an excited state by means of a hot filament, microwave plasma, high-frequency plasma, DC discharge plasma, or DC arc discharge, for example, has been developed.
The conventional CVD method mentioned above has necessitated a special device for the purpose of exciting the raw gas in such a manner as to permit synthesis of diamond. No matter which of the sources of excitation may be used, it is difficult to increase the surface area of diamond substances to be deposited.
The present inventors continued their studies with a view to developing a method liberated from the aforementioned drawbacks of the conventional CVD method and, as a result, developed a combustion flame method [Japanese Patent Public Disclosure HEI 1-282193].
This method, however, has the problem of practical utility because the diamond substances deposited by this method have only limited surface area. The present inventors, as the result of their studies continued with a view to developing a method for depositing diamond substances of a large surface area, have found that the deposition is easily accomplished by varying the parallel relative positions of a substrate and a burner thereby allowing the flame of combustion to contact the entire surface of the substrate. The invention embodying this knowledge has been applied for patent under Japanese Patent Application HEI 1-8091.
The method for synthesis of diamond by the use of combustion flame requires to burn a compound as a raw material for the deposition of diamond in such a manner as to give rise to a region of incomplete combustion in the atmosphere of combustion and place a substrate in the region of incomplete combustion or in a non-oxidizing atmosphere closely approximating the region. For the purpose of heightening the efficiency of deposition and the homogeneity of quality of diamond in the method of combustion flame, therefore, it is necessary during the combustion of the raw material for the formation of diamond to increase the amount of the raw material participating in the formation of the region of incomplete combustion to the fullest possible extent. From this point of view, the present inventors have continued their studies on the method of combustion flame in search of a way of enhancing the efficiency of deposition of diamond homogenizing the quality of deposited diamond.